The other need was for buffering the amplitude modulation (AM) signal. That eliminates one of the needs for the dual-rail supply. Okay, so using a common cathode diode allows us to bias our VCO with a positive voltage.

It seems like I’m able to eek out a little more performance below one volt and above eight volts. The capacitance of the diode varies from 87.4pf at 1 volt to 11.8pf at 8 volts. Looking up the MA4ST079CK-287T datasheet reveals the parameters. As far as I can tell, they don’t exist (though I think the author may still have some stock). The SMV1404-09 used in the N&V article, in contrast, is made out of unobtainium. I settled on the MACOM 4ST079CK-287T varactor diode, mainly because there was some guy selling them on ebay, 15 of them for $10. Whereas the author of the N&V article needed to use a negative voltage to control his VCO, I could get by with a positive voltage to control mine. The first change I made was to use a common cathode varactor diode instead of a common anode varactor diode. The case I had was a little bit small, and I decided I’d try to redesign the circuit to use only a single center-tapped 24 VAC transformer. The author’s RF generator used multiple transformers to deliver a variety of voltages. Design Modification – Single Supply Op-Amp I’ve got some good ideas from Nuts & Volts over the years, there’s some good inspiration there. If I recall correctly, buying a subscription gets you electronic access to back issues. I’ll show a few snippets of some of the modifications I made, but if you want to see the whole design or understand how it all works, I encourage you to buy the magazine. I’m not going to be providing the full schematics for the RF generator that I built, because that would be intruding on the property rights of Nuts and Volts and the author of the article.

Seemed like a great idea - rather than spend a hundred fifty bucks on an RF signal generator for aligning my radios, I could just build one and learn something in the process. It started when I read an article in the June 2014 issue of Nuts and Volts magazine.

Fm stereo generator circuit